Turbulent Coherent Structures and Circulation in Planar Contractions*

We measure time-resolved volumetric velocity fields in three 4:1 planar contractions with different contraction angles. We use four high-speed cameras and the Lagrangian particle tracking (LPT) technique for the measurements.  Experiments are conducted in a gravity-driven water tunnel with interchangeable 2-D contraction sections. The planar contractions have different streamwise lengths which we call the short, intermediate, and long contractions. An active grid generates turbulence which is the advected through the contractions to study its evolution. The grids operated in the random mode at 210 rpm resulted in Taylor Reynolds numbers of 170-220 at the inlet to the contractions.  Consistent with Prandtl’s theory for axisymmetric contractions, streamwise velocity fluctuations decay, while the transverse fluctuations are enhanced due to straining, for all three contractions. The strength of the preferential streamwise alignment of the coherent vortical structures is quantified using two measures, both based on the probability density function of cosine of angle between the structure and the mean flow, i.e. its cumulative probability and its moment of inertia. The intermediate contraction, which interestingly has a length almost equal to its inlet width, exhibits the strongest alignment. This has been reaffirmed by looking at the relative strengths of circulations Γ_x, Γ_y, and Γ_z computed in mutually perpendicular planes.